A node in an optical network may contain add-drop functionality in the form of a reconfigurable optical add/drop multiplexer (ROADM) device. The reconfigurable optical add/drop multiplexer (ROADM) may be arranged to connect optical fibers in the node such as to add or drop signals on any or all wavelengths carried by the optical fibers in a wavelength division multiplexing (WDM) communication system. This represents an important functionality for managing and reconfiguring data transfer in optical networks, in particular in order to effectively accommodate the increasingly growing bandwidth demands and increasingly unpredictably data traffic patterns in optical networks.
In view of this need, ROADMs have been widely studied and developed to increase the degree of flexibility for adding and dropping WDM channels. For example, multi-degree ROADM nodes have been introduced to allow service operators to route wavelengths between fibers crossing in a node in a number of directions. Colorless ROADMs allow transponders to receive and transmit signals on any wavelength employed by the ROADM. Directionless ROADMs allow transponders to receive signals originating from any input port and can forward signals to any output port. Contentionless ROADMs allow cross-connecting any wavelength channel between fibers, independent of other channels in use, as long as there is a free add/drop port and a free wavelength available on the respective fibers.
In other words, the provided flexibility of adding and dropping signals on WDM channels determines the scope of measures which are available for dynamically managing data traffic in optical networks in order to achieve an efficient or optimal allocation of network resources. Further, increased flexibility of adding and dropping signals on WDM channels can be applied to implement optical channel protection by transmitting data to a receiver via two redundant paths, namely a “working” path and a “protecting” path, respectively. Consequently, the “protecting” path provides a fallback position if for example a fiber used in the “working path” breaks. Similarly, “bridge and role” functionality can be implemented to securely transition a data transfer between optical fibers or transmission paths. For this purpose, the degree of flexibility of adding and dropping signals provided by the ROADM node must be sufficient to have a first channel signal transmission duplicated and additionally fed to a second channel. After establishing the signal transmission via both channels, the data transmission of the first channel can be shut down without risking interrupted connectivity.
FIG. 1 illustrates a conventional arrangement of the add/drop part of a reconfigurable optical add/drop multiplexer (ROADM) 10. Here, the reconfigurable optical add/drop multiplexer (ROADM) 10 offers optical channel protection by colorless, directionless and contentionless adding and dropping of channels on four incoming 12 and four outgoing fiber 14 connections. In this respect, connecting, selecting and multiplexing devices or apparatuses 46, 48 are arranged to connect the four incoming 12 and four outgoing fiber 14 connections to input and output amplifier stages 26, 24 of the reconfigurable optical add/drop multiplexer (ROADM) 10. The respective connecting, selecting and multiplexing devices or apparatuses 46, 48 are explained in further detail below in connection with FIGS. 5, 6 and 7. Eight optical line interfaces 16 are provided to add and drop signals at the node. The optical channel protection is provided by having signals to be added from the optical line interfaces 16 fed to a splitter 18 which duplicates the signal to a working path and a protection path, respectively. The duplicated signals are forwarded to switches 20 which in turn forward each of the signals to different combiner units 22. The respective arrangement consisting of switches 20 and combiner units 22 is sometimes denoted as aggregator switch. The different combiner units 22 combine signals for transmission via different channels or fibers and thus provide optical channel protection by transmission via separate working and protection paths. If required, amplifier stages 24 can be arranged as shown in FIG. 1 to amplify the combined signals prior to transmission via the outgoing fibers 14. Typically the signals are passing through a further amplifier before being sent out. In the drop direction, the signals arriving from the incoming fibers 12 are fed through an amplifier and demultiplexing or splitter unit directly or through amplifier stages 26 to splitters 28. FIG. 7 has illustrated for each incoming fiber 12 an amplifier and a subsequent splitter. The splitters 28 duplicate the signals and forward the same to a plurality of switches 30. Thus, each of the switches 30 receives a plurality of signals arriving from the incoming fibers 12 and is adapted to select one of these signals for further processing. The respective arrangement consisting of splitters 28 and switches 30 is sometimes denoted as multicast switch. The selected signal is then forwarded to a second switch 32 which is arranged to receive two selected signals from two switches 30. In this way, the drop direction arrangement is adapted to receive a signal via two paths, namely the “working” path and “protection” path, respectively. Then, the second switch 32 can select and forward the signal to be dropped via the “working” path or “protection” path, depending on the condition or requirements of the optical network. Thus, the signal selected by the second switch 32 is forwarded to the optical line interface 16 receiving the dropped signal.
It follows that optical channel protection is conventionally provided by a reconfigurable optical add/drop multiplexer (ROADM) 10 representing a complex arrangement including a large number of components. Consequently, a large number of components must be connected to optical line interfaces 16 by dedicated cabling via splitters 18 and switches 32. Thus, the conventional arrangement of a reconfigurable optical add/drop multiplexer (ROADM) 10 has the disadvantages that the large number of components require high cost of manufacturing and the device is subject to cumbersome installation cabling. Moreover, implementing and using a large number of components results in a system which is more prone to malfunction due to component failure.
Depending on the desired functionality of the optical channel protection, each of the optical line interfaces 16 must be connected accordingly during the installation process of the reconfigurable optical add/drop multiplexer (ROADM) 10. In particular, the optical line interfaces 16 must be connected to respective switches 20 and second switches 30 via splitters 18 and switches 32, requiring cumbersome, error-prone and costly cabling work. Moreover, such installation by cabling results in fixed connections which can generally not be changed remotely. It follows that the conventional approach of adding and dropping signals on WDM channels has a limited degree of flexibility which can impair or even disable options to implement and re-configure network management functionalities, such as for example optical channel protection and/or bridge and role functionalities.